Laser photocoagulator

ABSTRACT

A laser photocoagulator for treating a patient&#39;&#39;s eye comprises a laser, optical means for delivering the output beam from the laser to the desired location in the eye of the patient, and wherein the laser is operated to provide a multi-mode output beam. The use of the multi-mode output laser beam enables the treatment of certain eye diseases while insuring that damage to the cornea and other parts of the eye resulting from the passage of the laser beam is prevented.

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i [l9] in] 3,720,213 Hobart et al. 1M8l'Cll 13, 1973 s41 LASERPHOTOCOAGULATOR 3,456,651 7/1969 Smart ..128/303.1 [75] Inventors: JamesL. Hobart, Palo Alto; Steven :flbenmst it M Jarrett Los Altos Calif .13,467,098 9/1969 Ayres ..l28/303.l

[73] Assignee: ggfizrent Radiation, Palo Alto, OTHER PUBLICATIONS Yahr,W. 2., et al., Journ. of Assoc. for Advance- [221 1971 ment of Med.Instrumentation," Sept./Oct., 1966, pp. [21] Appl. No.: 113,026 28-31Primary ExaminerKyle L. Howell [52] U.S. Cl. ..128/395, l28/303.l,331/945 51 Int. Cl. ..A6ln 5/06 [58] Field of Search ..128/395398, 362,

128/303.1; 331/945 [57] ABSTRACT A laser photocoagulator for treating apatients eye [56] References Cited comprises a laser, optical means fordelivering the output beam from the laser to the desired location in theUNITED STATES PATENTS eye of the patient, and wherein the laser isoperated to 3,642,007 2/1972 Roberts etal. ..128/395 Provide a multi'mdeoutPut beam- The use 3559613 5/1972 Bredcmeieh" multi-mode output laserbeam enables the treatment 3,096,767 7/1963 Gresser et a]... of certaineye diseases while insuring that damage to 3,348,547 10/1967 Kavanaghthe cornea and other parts of the eye resulting from ,099 9/1969 Lotmarthe passage of the laser beam is prevented. 3,487,835 l/l970 Koester etal.. 3,617,927 11/1971 Pohl ..33l/94 5 8 Claims, 5 Drawing FiguresPATENTEDHAR 1.31m 3,720,213

sum 10F 2 TEMOOMODE E-FIELD I MULTl-ORDER MODE INVENTORS JAMES L. HOBARTSTEVEN M. JARRETT ATTORNEYS PATENTE'DHARI 31975 3,720,213

sum 2 or 2 FIG. 3

INVENTORS JAMES L. HOBART STEVEN M. JARRETT WW AZ ATTORNEYS LASERPHOTOCOAGULATOR BACKGROUND OF THE INVENTION This invention relates to alaser photocoagulator and more specifically relates to an improved laserphotocoagulator for treating disorders of the human eye.

Pioneering work in the early 1950s led to the first use ofphotocoagulation in the treatment of certain problems of the eye.Instruments using incoherent light, primarily Xenon-arc-discharges, weredeveloped and have been used with some success. The development of thelaser in 1960 made it possible to consider laser light sources insteadof incoherent sources. The laser offered the advantages of higher power,smaller focus spot size and better absorption in the eye. One use oflaser photocoagulation is in treating retinal detachments in the humaneye by fusing the retina of the eye to the cohoroid. Laserphotocoagulation is used to destroy tumors, to prevent the spread ofdisease and in many other ways known to the medical profession.

Since photocoagulation destroys the eye tissue in the area beingcoagulated an effective coagulation apparatus must focus the intenseenergy only upon a carefully sized and selected portion of the eye whichis to be treated.

For example, in treating the macula portion of the eye, it is imperativethat extremely small spot sizes be utilized in order to preventinadvertant destruction of the fovea. The fovea is located within themacula and is substantially responsible for the reading functionperformed by the eye. In order to insure the safety of the fovea, spotsizes in the ranges, for example, of 40 to 50 microns are required.

It can be shown that using a photocoagulator with a laser operating inthe usual fundamental or TEM mode, the spot size on the retina is givenapproximately by the relationship 1. Spot size 2 2 F -l. where F is thefocal ratio of the eye lens and A is the wave length of the laser beam.

This also assumes that a Goldman type contact lens is used to neutralizethe refraction effect of the eye's cornea and lens. The focal ratio F isgiven by 2. F fL/D where fL is the focal length of the eye,

and D is the diameter of the circular area of the lens which isilluminated by the beam.

Equation (2) may also be expressed as 3. D fL/ F in such applications asdescribed above where, for example, a 50 micron spot size is desired,the diameter A of the laser beam can be calculated easily. It is firstnecessary to calculate F which can be found by rearranging equation (I),and where l. for an argon laser output is approximately 0.5 microns:

4. F=Spot size/2a 50 microns/2.5 microns 50 And therefore, substitutingF in equation (3), and where [L is equal to approximately 1.7 cm in theordinary eye:

Unfortunately, a laser beam providing a 40 or 50 micron spot havingsufficient powers to be useful to cause coagulation can damage thecornea where the diameter of the beam at the point where it passesthrough the cornea is 0.340 mm as calculated above. This damage iscaused because the energy density at the cornea is too great.

In particular, slight burning of the cornea due to absorption by thecornea itself, opacities in the cornea or particles on the surface,sometimes results. This problem was not discovered at an earlier datebecause earlier laser photocoagulators did not have as much laser poweravailable, and consequently, although the problem was inherent, it wasnot realized.- A second roblem caused by the high energy density boththrough the cornea and through the ocular media of the eye is calledthermal-defocusing or blooming. Thermalblooming results in larger spotsizes than desired on the retina. T hermal-blooming is caused by changesin the refractive index of the lens of the eye and other parts of theeye due to heating thereof. Heating of the cornea and lens results in asmall change in the index of refraction in the cornea and lens andproduce a thermal lens effect. This effect then causes the beam to bedefocused resulting in a spot size on the retina which may be 3 to 5times larger than would occur without thermal-blooming. As a result theenergy density at the retina may be insufficient to cause coagulation.The problem is particularly acute in older patients where there is moreabsorption in the cornea and lens.

in order to reduce the energy density through the cornea and ocularportions of the eye without reducing the energy imparted at the point oftreatment, the solution is to increase the area of the beam at the pointit passes through the cornea. Thus instead of the 0340mm diameter beamin the case calculated above, a 1.0 mm diameter beam would, for example,permit a beam having lower, safer, energy density through the eye. Infact since the energy density is a function of the area of the beam, andsince the area of the beam is a function of the square of the diameterof the beam, the above increase in D of 3 to 1 would result in an energydensity reduction of 9:1. And, since as a practical mater, it is*oftendifficult to achieve a value of D as small as 0.340 mm as calculatedabove, providing a 1.0 mm diameter beam to the cornea would achieve areduction in the energy density through the cornea of greater than 9: 1.

If one substitutes a value of 1.0 mm for D in the above equations,however, it will be found that the resulting spot size is only 17microns. This is significantly smaller than the 40 to micron spot sizethat was the original objective.

To provide a larger spot size while maintaining a sufficiently largervalue of D, the common method of increasing the spot size to a sizewhich can safely be ad- '"rninistered to the eye has been to introducean optical system for focusing the beam from the laser to a point beyondthe surface of the retina being treated. This means that the size of thebeam at the retina is larger than the spot size at the focal point ofthe beam. Unfortunately, this procedure has several serious drawbacksfor the smaller spot sizes.

A main problem resulting from this approach is that when focusing in thevitreous, to treat a blood vessel, for example the focal point of thelaser beam extends past the vessel onto the retina. With a larger valueof D, the depth of field of the beam decreases and consequently thoseregions upon which the beam is focused as the retina above aresusceptible to damage caused by the high intensity small spot size.

Several avenues were explored to solve these problems. In each case thepurpose was to decrease the techniques to reconstruct an image of thedesired size on the retina and a fiber optic technique to create animage of the desired spot size which could then be reimaged on theretina. None of these solutions proved feasible however.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to provide an improved laser photocoagulator.

Another object of the invention is to provide a laser photocoagulatorwith a laser arranged to operate to provide a multi-order output beamtherefrom.

Another object of the invention is to provide a laser photocoagulatorwhich can be used to provide small spot sizes on that portion of the eyebeing treated which does not result in damage to the cornea and lens ofthe eye.

Another object of the invention is to provide an improved laserphotocoagulator in which the output beam to the eye is of a sufficientlylow energy density so that thermal-blooming does not occur to enlargethe spot size.

Another object of the invention is to provide a method for treatingcertain eye diseases not otherwise treatable with other laserphotocoagulator systems.

ln accordance with the invention the above-mentioned problems areeliminated by changing the properties of the laser beam in such a way toachieve the desired larger spot size on the cornea for a given spot sizeon the retina. The latter is accomplished by providing an output beamhaving a higher beam divergence than that provided by a laser operatedin the normal fashion.

In particular, the laser is modified to operate in a higher ormulti-order transverse or spatial mode which results in an output beamangle of divergence approximately 2.3 times that of the normal, orfundamental, lowest-order transverse mode (TEM In order to convey thealtered laser beam to the eye, appropriate optics must be employed inthe photocoagulator system. The selection of these optics involve theuse of techniques familiar to those skilled in the design of laseroptical systems.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged cross-sectionalview of a human eye.

FIG. 2 is an elevational view of a laser photocoagulator system inaccordance with the present invention.

FIG. 3 is an elevational view, partially in section, of the deliverysystem of the photocoagulator system shown in FIG. 2.

FIG. 4A is a cross-sectional representation of the energy distributionof a TEM mode laser beam; FIG. 4B is a cross-sectional representation ofa multi-order, multi-mode laser beam.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The difiiculties of prior artlaser photocoagulators using a laser in its normal fundamental modeoperation can best be seen by reference to FIG. 1 showing a human eye 10including a cornea l2, and lens 14 and a retina 16. As is standardpractice, a Goldman corneal contact lens 18 is placed on the comes 12.The contact lens 18 eliminates the refractive effects of the cornea 12and the lens 14 and thereby provides an effective refractive indexthrough the eye of approximately one.

As can be seen in FIG. 1 the focal length of a typical eye is 1.7centimeters. When, as explaineclabove, a 40 to 50 micron beam size isrequired, as for example to treat areas of the macula, a fundamentalorder laser beam 20 typically creates a spot size 22 of about 17 micronson the retina 16, for a beam having a sufficiently large value of D (forexample, 1.0 mm) as it passes through the lens 14 that neither burningof the cornea or thermal-blooming occurs. By providing an additionallens (not shown), the usual practice is to provide a beam 24 which has afocal point 26 beyond the retina 16 of the eye. With the focal point at26 the spot size 22 at the retina is increased to the desired size,typically a minimum of around 40 microns. As explained above, thisresults in undesirable difficulties.

In accordance with the present invention a laser beam is provided havinga high order or multi-order mode. As will be explained in more detailsubsequently, the geometry of a beam of a higher order mode is such thatspot sizes as low as 40-50 microns are achievable wherein the energy atthe point of treatment is sufficient to cause coagulation and at thesame time the area of the beam through the lens is sufiiciently large(i.e., large value of D0 that the energy density of the beam is lowenough to insure safety to the cornea and also prevent thermal-bloomingReferring now to FIG. 2, there is shown an overall view of a laserphotocoagulator, such as the slit lamp photocoagulator described incopending patent application Ser. No. 41,505, entitled Slit LampPhotocoagulator, filed May 28, I970, by Vassiliadis et al. A laser 30 isprovided in a console 32 which also encloses a control box 34 and othernecessary electronic equipment 36. In one embodiment laser 30 is acontinuous wave argon laser, such as a Model 52A laser manufactured byCoherent Radiation of Palo Alto, California, the assignee of the presentinvention. Although other continuous wave lasers could be substituted inthe present invention such as a Krypton laser or an argon/krypton laser,such as Coherent Radiation Model 52K and 52MG respectively, the argonlaser has certain advantages. The high absorption of the argon laserblue-green wave length by the hemoglobin in the eye makes is possible totreat vascular diseases of the eye effectively. Also the high absorptionof the argon laser wave length by the pigments in the eye and the hightransmission by the ocular media make is possible to treat diseases withless power than that required by a Xenon-arc photocoagulator.

An argon laser has high stability which makes possible very accuratecontrol of the dosage of each exposure. Further, as would be true of anycontinuous laser, the continuous wave nature of the argon laser allowsexposures to be made and do not cause shock waves usually associatedwith, for example, pulsed ruby photocoagulators.

- of slit lamp orientation. The slit lamp 42 is a standard The output ofthe laser 30 is coupled through a shutter assembly 38, through anarticulated arm 40 to a slit lamp 42. The shutter 38 is adapted to bemoved into and out of the path of the laser beam by a rotary solenoid(not shown). The purpose of this shutter assembly 5 38 is to permit avery small fraction of the laser beam to be transmitted therethroughwhile the shutter is in the path of the laser beam. This small fractionof the laser beam acts as an accurate aiming beam in the instrumentsince it proceeds along the same path that the full beam will followwhen the solenoid is actuated to the shutter assembly and is rotated outof the path of the laser beam. Thus the laser beam acts as its ownaiming beam and the aiming beam can be seen impinging on the targetsite, say on the retina, for example. The ophthalmologist thus can seenexactly where the photocoagulation will take place. Further detailedexplanations of this feature of the photocoagulator i described in theaforementioned patent application.

The shutter assembly 38 can be either automaticall set for apredetermined length of time or can b manually operated by the operator.The laser beam i introduced into the optics of the slit lamp 42 in an accurately maintained directional position independent type slit lamp suchas, for example, that manufactured by Carl Zeiss, Inc. in West Germany,The slit lamp 42 comprises a binocular optical viewing arrangement,generally indicated by reference numeral 44 and a light source 46 forilluminating the eye under observation.

Referring now to FIG. 3 there is shown a side view partially in sectionof the slit lamp 42. The slit lamp 42 permits magnetic binocular viewingof portions of a patients eye indicated by reference numeral 10. 3 5

Also a part of the standard slit lamp is the light source 46 whichsupplies light to a mirror 50 which reflects the light upward to a prism52. The prism 52 reflects the illuminating light through the Goldmanlens 18 onto the patients eye 10. A standard slit lamp also includes apositional control lever 54 which may be used to adjust the orientationof the slit lamp.

The laser beam is coupled into the slit lamp from the articulated arm 40through an opening 56. The beam goes through lens f5 and then throughone of a plurality 5 of lenses f4 forming a lens turret.

The lens wheel or turret f4 may contain any number of additional lensessuch as, for example the five lenses f41 f45. The lenses are mounted forrotation about an axis (not shown). In this manner, any of the lensescarried by the turret f4 may be moved into the path of the laser beam.

Each lens in the turret is specially designed and provides a specificsize of beam focus at the same fixed position in space that iscoincident with the focal plane 55 of the binocular viewing optics ofthe slit lamp or beyond the focal plane or rear of the eye. In this waydifferent size exposure sites can be obtained at the target site alwayscoincident in space with the view of the ophthalmogist or operator. Thatis, the size of the exposure at the target can be varied withoutchanging the focus.

Each of the individual lenses in the turret is designed such that for anincident nearly columinated beam of certain cross-section, a specificimage size is formed at the focus. Through the use of these speciallenses the laser beam is always coincident with the focus view of theophthalmogist and yet different focus beam spot sizes are obtained.

In accordance with the invention the laser 30 shown in FIG. 2 isconstructed to operate in a higher mode operation. As explained above,the normal operation of a laser such as an argon ion laser is thefundamental or low order mode such as TEM The TIEZM, mode is illustratedgraphically in FIG. 4A showing the energy distribution of a laser beamoperated in the TEM mode. This mode provides the smallest beam size andhence a high energy density. This is normally a desirable mode for mostapplications using a laser. In fact, in constructing a gas laser, onenormally attempts to operate the laser at the lowest mode possible.

The cross-section of energy distribution of a higher order, multi-ordermode laser beam is shown in FIG. 48. It can be seen that the energydensity is more evenly distributed across the cross-section of the beamthan in a case of the gaussian distribution of the TEM mode of FIG. 4A.

The theory and specifics with respect to laser beam modes are describedin much detail in a book entitled Gas Lasers by Arnold L. Bloom,published by John Willy and Sons, 1968. In particular Chapter 3beginning at page 69, provides additional information about laser beammodes.

It can be shown mathematically that the mode structure of the laser beameffects the focus spot size for a given optical system such as theoptical system of the eye, and in fact, it can be shown that a higherorder mode wave front propagating this space will behave approximatelythe same as a propagating wave front of a fundamental mode beam with theexception that the beam waist has a cross-sectional diameterapproximately 2.3 times those of a gaussian beam.

It was explained earlier that a beam 20 of a fundamental mode provides aspot size of approximately 17 microns where the beam diameter isincreased to 1.0 mm and where defocusing is not used. Also explained wasthe fact that this size is too small to insure safe operation on the eyeor to prevent thermal-blooming. With the multi-mode beam substituted fora fundamental mode beam, the spot size is approximately 2.3 times thatfor the fundamental mode beam. Thus, instead of a l7 micron spot size, aspot size of approximately 39 microns results, which is in the desiredoperating range. These same calculations can be performed for a varietyof desired spot sizes, and each case the spot size is approximately 2.3times that calculated for the guassian wave front.

Thus, operating the laser 30 in a higher order mode, it is possible toachieve a spot size on the retina or other part of the eye which isbeing treated which is approximately 2.3 times the size of that for aphotocoagulator using a conventionally operated low mode beam. With alaser operated in the higher mode region it is possible therefore, toprovide a spot size large enough to insure that no damage will occur toareas of the eye adjacent to those portions being treated while at thesame time maintain a beam having a sufficiently large cross-sectionalarea as it passes through the cornea and lens to insure that no damagewill occur thereto, and to maintain the energy density at a sufficientlylow level that thermsl-bloomin g will not occur.

An operational laser providing multi-mode output beams can beconstructed easily by interchanging the reflectors constituting theoptical resonator of the usual laser with a set of reflectors whichpromote higher mode oscillation rather than low order mode oscillation.For example, the mirrors used in the resonators for normal modeoperation of Coherent Radiation Model 52 argon laser comprise a fivemeter radius output coupler and a flat total reflector, comprising along radius-flat type resonator. To provide a multi-mode output, theabove reflectors are removed and 120 centimeter radius mirrors aresubstituted for both the total reflector and the output coupler therebyforming a near confocal type resonator. Further explanation of the typesof resonators used in lasers can be found in the book cited aboveentitled Gas Lasers by Bloom beginning at page 74.

Details of the specific lens configuration used to deliver themulti-order mode laser beam to the eye are described in the followingchart. The lenses referred to are shown generally in FIG. 3.

Focal Length (in cm) Distance between Lenses cordance with the presentinvention can be maintained at a large cross-sectional area, it ispossible to increase the amount of optical power into the eye. At thesame time the spot size is sufficiently large that it can safely beadministered to the eye without damage thereto.

The use of a multi-order mode beam also has several other advantages.The ability to use larger power beams with multi-mode operation meansthat less power is required in the operation of the laser which insuresa longer laser life. Another advantage is the multimode operationrequires less sensitive alignment of the mirrors forming the opticalresonator. This means that doctors and other physicians and techniciansusing the equipment, who are normally unskilled in laser technology,have less difficulty in maintaining the laser operation.

We claim:

1. Laser photocoagulator for treating a patient's eye comprising:

a. a gaseous laser;

b. optical means for delivering the output beam from said laser andfocusing the same on a desired treatment location in the eye of thepatient; and

c. means for operating said gaseous laser to provide a high ordertransverse mode out ut beam. 2. Laser photocoagulator as in c arm 1wherern said laser includes an optical resonator assembly and whereinsaid operating means comprises a near confocal optical resonator.

3. Laser photocoagulator as in claim 1 wherein said laser comprises acontinuous wave laser.

4. Laser photocoagulator as in claim 1 wherein said laser comprises anargon laser.

5. Laser photocoagulator as in claim 1 wherein said laser comprises anargon/krypton laser.

6. Laser photocoagulator as in claim 1 wherein said laser comprises akrypton laser.

7. Laser photocoagulator as in claim 1 wherein said operating meansadditionally provides a multi-order transverse mode output beam.

8. Laser photocoagulator for treating a patients eye comprising:

a. a gaseous laser;

b. optical means for delivering the output beam from said gaseous laserto the eye of a patient; and

c. means for providing a relatively small focused multi-transverse modelaser beam spot to the area of treatment within the eye, wherein theenergy imparted at said area is sufficiently high to insure coagulationthereof, and wherein the energy density of the beam through the eye issufficiently low to prevent damage to the eye and preventthermalblooming.

1. Laser photocoagulator for treating a patient''s eye comprising: a. agaseous laser; b. optical means for delivering the output beam from saidlaser and focusing the same on a desired treatment location in the eyeof the patient; and c. means for operating said gaseous laser to providea high order transverse mode output beam.
 1. Laser photocoagulator fortreating a patient''s eye comprising: a. a gaseous laser; b. opticalmeans for delivering the output beam from said laser and focusing thesame on a desired treatment location in the eye of the patient; and c.means for operating said gaseous laser to provide a high ordertransverse mode output beam.
 2. Laser photocoagulator as in claim 1wherein said laser includes an optical resonator assembly and whereinsaid operating means comprises a near confocal optical resonator. 3.Laser photocoagulator as in claim 1 wherein said laser comprises acontinuous wave laser.
 4. Laser photocoagulator as in claim 1 whereinsaid laser comprises an argon laser.
 5. Laser photocoagulator as inclaim 1 wherein said laser comprises an argon/krypton laser.
 6. Laserphotocoagulator as in claim 1 wherein said laser comprises a kryptonlaser.
 7. Laser photocoagulator as in claim 1 wherein said operatingmeans additionally provides a multi-order transverse mode output beam.